Physics
Newton's laws of motion — quick study summary
AP PhysicsGCSE PhysicsIB Physics SL/HL
Newton's three laws describe the relationship between forces and the motion of objects. They form the foundation of classical mechanics and explain everything from cars stopping at red lights to rockets reaching orbit.
Key points
- 1st law (inertia): an object stays at rest or moves at constant velocity unless acted on by a net force
- 2nd law: F = ma — net force equals mass times acceleration
- 3rd law: for every action there is an equal and opposite reaction
- Forces are vectors — they have magnitude AND direction
- If net force = 0, the object is in equilibrium (still or constant velocity)
Practice quiz
Click each question to reveal the answer.
1. A 2 kg object accelerates at 3 m/s². What is the net force on it?
Answer: 6 N
F = ma = 2 × 3 = 6 N.
2. Which Newton's law explains why a passenger jerks forward when a car brakes?
- 1st law (inertia)
- 2nd law (F = ma)
- 3rd law (action-reaction)
- Conservation of momentum
Answer: 1st law (inertia)
3. If you push a wall with 50 N of force, what force does the wall push back with?
Answer: 50 N
Newton's 3rd law — equal and opposite reaction.
Last reviewed: May 2026